Zeolites or molecular sieves are three-dimensional crystalline alumina-silicates with internal pore structure having both cation exchange capacity and adsorption capacity. The cation exchange capacity of zeolites is related to the aluminum content and pore size, the aluminum atom carrying a unit negative charge in the crystalline framework and thus determining the cation exchange capacity. If the hydrated radius of a cation is too large for diffusion through the pores, then exchange capacity and rate for that cation will be low. The adsorption capacity of zeolites is related to the pore size and the size of the adsorbate molecule. Materials which can be diffused into the pores will be adsorbed while larger ones will not.
Zeolites may be synthesized from concentrated silica-alumina gels under basic conditions such as described in U.S. Pat. No. 2,882,243. According to U.S. Pat. No. 3,663,165, a zeolite molecular sieve catalyst has been synthesized from kaolin clay by calcining the kaolin at 1800.degree. F. for 2 hours to form m-kaolin followed by two treatments with aqueous sodium hydroxide. The first treatment is carried out at 100.degree. F. for 12 hours while the second is carried out at 180.degree. F. for 12 hours.
Certain zeolites do occur as mineral deposits in certain parts of the world. These mineral zeolites are usually found with varying amounts of other impurities and generally have low aluminum content. A particularly abundant mineral or natural zeolite is clinoptilolite having the formula Na.sub.x (AlO.sub.2).sub.x (SiO.sub.2).sub.y.ZH.sub.2 O wherein x=5 to 7, y/x=4.5 to 5.5 and Z=1 to 30.
According to a Russian article by A. Yu. Krupennikova, et al published by The P. G. Melikishvili Institute of Physical and Organic Chemistry of the Academy of Sciences of the Georgian S.S.R. and entitled Phase Transitions in the Recrystallization of Clinoptilolite, clinoptilolite has been subjected to caustic treatment to produce sodalite or phillipsite apparently in accordance with the equation: EQU 2Na.sub.6 (AlO.sub.2).sub.6 (SiO.sub.2).sub.30.24H.sub.2 O+96NaOH(Aq).fwdarw.Na.sub.12 (AlO.sub.2).sub.12 (SiO.sub.2).sub.12.27H.sub.2 O+48Na.sub.2 SiO.sub.3 (Aq)+69H.sub.2 O (I)
As is apparent, reaction (I) suffers from a dramatic weight loss and is more suitable for the production of Na.sub.2 SiO.sub.3. In fact, each pound of clinoptilolite introduced into the process produces but 0.4 pounds of zeolite A and for each pound of zeolite A produced there is an accompanying 0.57 pound of water which must be vaporized in order to isolate zeolite A.
It is a principle object of the present invention to provide a relatively simple process which produces zeolite A from clinoptilolite while achieving a high yield of zeolite A per pound of clinoptilolite introduced into the process and requiring removal of a minimal amount of water in order to isolate the zeolite.